Pulmonary Function Tests

Pulmonary function testing can be done in a pulmonary function laboratory. Simpler tests can also be done in a physician's office or in the home. Testing most often is used to identify airflow abnormality, reduced lung volume (restrictive abnormality), diffusing capacity changes (as in interstitial disease), and blood gas abnormality (too much carbon dioxide, too little oxygenation). The following tests are commonly performed.

Spirometry includes a number of simple measurements. Vital capacity (VC) measures the total volume of air one can breathe out completely after inhaling a full breath. VC is usually done forced, as fast as possible, and is known as FVC. When this fast forced expiration is performed, the volume breathed out in the first second is known as FEV1. The ratio of FEV1/FVC is an important parameter for assessing airflow and possible airflow obstruction. (Airflow obstruction is often defined as FEV1/FVC of less than 70%.) VC is sometimes done slowly and is called SVC. SVC avoids dynamic airway compression and may yield a larger VC than FVC, but cannot be used to assess airflow. The maximum airflow, or peak expiratory flow (PEF), and other airflow measurements can be made from the same forced expiration. These tests can be done while standing, sitting or lying supine. A significant change in VC when measured in a different position may be due to respiratory muscle (particularly diaphragm) weakness.

Tests used to evaluate neuromuscular diaphragmatic function include VC (either sitting or lying supine), maximum inspiratory force (MIF), maximum expiratory force (MEF), maximum voluntary ventilation (MVV), and nocturnal oximetry. MIF and MEF are measured by breathing in and out with maximal effort, through a closed mouth tube attached to a pressure measuring device. This measurement reflects inspiratory and expiratory muscle power. MVV is measured by breathing in and out deeply and rapidly for a short time. MVV is a more global measurement of maximal breathing capacity and respiratory muscle endurance.

Lung volume testing measures VC and the residual volume (RV) of air still in the lung after breathing all the way out. This allows calculation of the total lung capacity (TLC), which may be reduced due to causes such as neuromuscular disease, kyphoscoliosis and pulmonary fibrosis.

Diffusing capacity (DLco) is a measurement of gas transport (transfer) across the lung's alveolar air sacs and capillaries into the blood. Causes of a reduced DLco include interstitial pulmonary disease and fibrosis.

Blood oxygen, carbon dioxide, bicarbonate and pH are measured from an arterial blood specimen. Arterial blood gas (ABG) measurements can be done at rest or with exercise, breathing room air or oxygen. The oxygen saturation can be measured noninvasively using an oximeter. One type of oximeter has a memory module so that it can record 8-12 hours of oxygen and pulse rate data; this is useful to evaluate data noninvasively overnight (nocturnal oximetry).

Sleep studies are usually performed in a sleep laboratory with an overnight study recording multiple variables simultaneously to assess sleep disorders (such as sleep apnea or underventilation). These studies include EEG (brain wave), ECG (electrocardiogram), airflow measured at nose and mouth, oximetry, muscle measurements of chest and abdomen, and often a video record of sleep movements. As respiratory muscle weakness develops in neuromuscular disease, night-time underventilation and a drop in oxygen saturation may be an early finding. Sometimes nocturnal oximetry alone may provide sufficient information; this can be done at home.

A polio survivor with an abnormal sleep study should consult a pulmonologist experienced in neuromuscular diseases to determine the best treatment (particularly CPAP versus assisted ventilation for respiratory muscle weakness).

A polio survivor scheduled for pulmonary function tests should at least have spirometry with SVC (if possible sitting and supine), FVC, FEV1; MIF and MEF; and oximetry on room air. Nocturnal oximetry should also be considered since it is a simple useful test for abnormalities that often occur first during sleep.

Home or office monitoring of VC, PEF, and oximetry can be done with simple devices. For home monitoring, people often use a peak flow meter or an incentive spirometer (both are relatively inexpensive), to assess whether they are stable or worsening, especially during a respiratory infection. A nurse or respiratory therapist can easily measure the oxygen saturation on home visits with a small portable oximeter.